1. Field of the Art
The present invention relates to a process for producing spherical composite cured melamine resin particles in which colloidal silica is localized near the surface thereof. Further, the present invention relates to a process for producing the spherical composite cured melamine resin particles the surface of which is coated with inorganic compound particles. The spherical composite cured melamine resin particles of the present invention are particularly excellent in water resistance, and have good resistance to solvent attack, good heat resistance and a characteristic that the particles have a narrow particle size distribution. The particles are used suitably for several polishing agents, paints, ink, flatting agents, resin fillers, slipping improvement agents for resin films, column packings, anti-wearing agents, spacers for liquid crystal displays, light diffusion agents for light diffusion sheets, electrophoresis displays, hard coating agents for touch panels, toners, electrodes for solar cells, photocatalysts for decomposing water, optical materials, magnetic materials, electrically-conductive materials, flame retardants, paper-making materials, fiber-treating materials and the like.
2. Description of the Related Art
As methods for producing spherical cured melamine resin particles, several methods have been proposed. JP-A-50-45852 discloses a method comprising introducing a precondensate obtained by reacting benzoguanamine, melamine and formaldehyde in a given pH region into a hydrophilic polymer protective colloid under stirring to obtain an emulsion, adding a curing catalyst, such as an acid thereto and carrying out a curing reaction. In addition, JP-A-62-68811 discloses a method comprising condensing a hydrophilic precondensate of melamine and/or benzoguanamine and formaldehyde, and curing in the presence of a suspension of an alkylbenzenesulfonic acid having a C10–C18alkyl group, in an aqueous solution containing an anionic or nonionic surfactant. These methods provide spherical cured melamine resin particles with a narrow particle size distribution. However, these methods have a problem that the water resistance of the resin particles is liable to be lowered because the resultant cured resin particles are contaminated by the water-soluble protective colloid and surfactants used as dispersants.
JP-A-52-16594 describes a method comprising adding fine particulate silica having a particle size of 0.05 μm or less and a curing catalyst to an emulsion of an uncured benzoguanamine resin colored with a dye, and curing the resulting mixture in a emulsified state. As the method uses a water-soluble protective colloid when the emulsion of benzoguanamine resin is synthesized, it has a problem that the water resistance of the resin particles is liable to be lowered as mentioned above. The fine particulate silica is used as a dispersion improvement agent in order to prevent agglomeration of cured benzoguanamine resin particles.
Also is proposed a method of producing spherical cured melamine resin particles with neither protective colloids nor surfactants. JP-A-50-151989 discloses a method of condensing a component to be condensed, which is selected from phenols, ureas and aromatic amines, with an aldehyde compound in an inert organic medium in the presence of a suspension of inorganic powders. The method uses an organic solvent as a reaction solvent, therefore it raises manufacturing cost, increases environmental burden and is not an economical method. JP-A-62-10126 describes a method of reacting melamine with an aldehyde compound in an aqueous medium in the presence of an emulsion of a basic catalyst and an essentially water-insoluble inorganic salt, such as calcium fluoride, magnesium fluoride or strontium fluoride. As the cured resin particles obtained by the method are coated with essentially water-insoluble inorganic salts, they have good water-resistance. However, the publication merely discloses cured resin particles having a particle size of about 10 μm in Example 1, and it does not disclose that particles having a particle size of submicron order are obtained. Therefore, the method has a problem in ranges of the control of particle size. Thus, it is desired to develop a method in which the particle size can be controlled in a broad range in order to extend the scope of use to which cured resin particles are applied.
U.S. Pat. No. 3,845,006 discloses a method comprising mixing and reacting an aqueous solution of polysilicic acid with an aqueous solution of aminoresin to produce polysilicic acid-aminoresin copolymer particles being spherical, and having holes and particle size of 1 to 50 μm. On the other hand, spherical composite cured melamine resin particles obtained by the present invention described below are different with particles described in the above-mentioned US Patent, in that in the present invention, colloidal silica and a melamine resin are not copolymerized uniformly, divided into phases and the resin particles are complicated as mosaic polymers.
In addition, the polysilicic acid is a highly hydrolyzed active silica having a molecular weight of 100,000 or less and particle size of less than 50 Å(5 nm). Colloidal silica has a molecular weight more than the polysilicic acid and particle size of 50 Å or more (see, Ralph K. Iler; The Chemistry of Silica, John Wiley & Sons, Inc, 1979, p. 11). Consequently, the polysilicic acid disclosed in the above-mentioned US Patent is different from the colloidal silica in the present invention.
U.S. Pat. No. 3,846,453 describes a method comprising mixing and reacting an aqueous solution of sodium silicate with an amino compound and formaldehyde to produce aminoresin-silica composite particles having primary particle size of 0.05 to 0.3 μm and cluster particle size of 1 to 10 μm. However, the method using the aqueous solution of sodium silicate does not produce composite particles stably in the test conducted by the present inventors (Comparative Example 3 of the present application).
As mentioned above, the conventional methods have problems that in case where a water-soluble surfactant is used when spherical cured melamine particles are synthesized, water resistance of the resulting resin particles is lowered, and that in case where an inorganic compound is used, it is difficult to control particle size of the resulting resin particles in broad ranges.